ENRTE WX VERY UNFRIENDLY TO SMA PLT WHEN HE FORGETS TO TURN ON HIS PITOT TUBE HEATER IN ICING CONDITIONS. ALTDEV LATER OCCURS.

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I WAS ON AN IFR FLT PLAN FROM HGR TO CHO. I WAS LEVEL AT 6000 FT WHICH WAS ALSO THE MEA ALONG V143. I BEGAN TO ACCUMULATE LIGHT RIME ICE. I SAW DOME CLRING ABOVE AND ASKED TO CLB TO 8000 FT THINKING I WOULD BE OUT OF THE CLOUDS. UPON REACHING 8000 FT I STILL REMAINED IMC AND CONTINUED TO DEVELOP LIGHT ICE (NOW ABOUT 1/2 INCH). I ASKED TO DSND BACK TO 6000 FT AND WAS CLRED TO 6000 FT. DURING THE DSCNT MY AIRSPD DECREASED TO 0 KTS. I REALIZED THAT I HAD FORGOTTEN TO TURN ON THE PITOT HEAT ALLOWING ICE TO COVER THE PITOT TUBE. WHAT I ALSO FORGOT WAS THAT THE STATIC PORT ON THE SMA ACFT IS ALSO ON THE PITOT TUBE. I TURNED ON THE PITOT HEAT AND CONTINUED TO DSND. I LOOKED AT THE ALTIMETER AND IT READ 6600 FT SO I CONTINUED TO DSND. THE ICE MELTED FROM THE PITOT TUBE AND THE ALTIMETER SPUN RAPIDLY DOWN TO 5700 FT; 300 FT BELOW MY ASSIGNED ALT AND THE MEA. I QUICKLY ADDED PWR AND STARTED TO CLB BACK TO 6000 FT. AT THIS TIME CTR CALLED AND ASKED MY INDICATED ALT. I RESPONDED 5700 FT AND CORRECTING. CTR MADE NO FURTHER COMMENTS ABOUT MY ALT. WITHIN A FEW MINS I WAS CLRED TO 5200 FT DIRECT CHO. THE REST OF THE FLT WAS UNEVENTFUL. I WILL ADD THE PITOT HEAT TO THE CHKLIST SO I WON'T FORGET IT NEXT TIME I FIND MYSELF IN ICING CONDITIONS. I WILL ALSO BE MORE ATTENTIVE TO THE FREEZING LEVEL WHICH WAS AROUND 6000 FT; SAME AS THE MEA. AS A CFI I PLAN TO EDUCATE MY STUDENTS WITH THIS STORY; HOPEFULLY PREVENTING FUTURE FLTS LIKE THIS THAT MIGHT NOT END SO WELL.